A carbon nanotube can be visualized as a sheet of hexagonal graph paper rolled up into a seamless tube and joined. Each line on the graph paper represents a carbon-carbon bond, and each intersection point represents a carbon atom.
In general, carbon nanotubes are elongated tubular bodies which are typically only a few atoms in circumference. The carbon nanotubes are hollow and have a linear fullerene structure. The length of the carbon nanotubes potentially may be millions of times greater than their molecular-sized diameter. Both single-walled carbon nanotubes (SWNTs), as well as multi-walled carbon nanotubes (MWNTs) have been recognized.
Carbon nanotubes (also referred to as “CNTs”) are currently being proposed for a number of applications since they possess a very desirable and unique combination of physical properties relating to, for example, strength and weight. Carbon nanotubes have also demonstrated electrical conductivity (Yakobson, B. I., et al., American Scientist, 85, (1997), 324-337; and Dresselhaus, M. S., et al., Science of Fullerenes and Carbon Nanotubes, (1996), San Diego, Academic Press, 902-905). For example, carbon nanotubes conduct heat and electricity better than copper or gold and have 100 times the tensile strength of steel, with only a sixth of the weight of steel. Carbon nanotubes may be produced having extraordinary small size. For example, carbon nanotubes are being produced that are approximately the size of a DNA double helix (or approximately 1/50,000th the width of a human hair).
Considering the excellent properties of carbon nanotubes, they are well suited for a variety of uses, such as building computer circuits, reinforcing composite materials, and even to delivering medicine. In addition, carbon nanotubes may be useful in microelectronic device applications, which often demand high thermal conductivity, small dimensions, and lightweight. One application of carbon nanotubes that has been recognized from their use in flat-panel displays uses electron field-emission technology (since carbon nanotubes can be good conductors and electron emitters). Further applications that have been recognized include electromagnetic shielding, for cellular phones and laptop computers, radar absorption for stealth aircraft, nano-electronics (including memories in new generations of computers), and use as high-strength, lightweight, multifunctional composites.
However, attempts to use carbon nanotubes in composite materials have produced results that are far less than what is possible because of poor dispersion of nanotubes and agglomeration of the nanotubes in the host material. Pristine SWNTs are generally insoluble in common solvents and polymers, and difficult to chemically functionalize without altering the nanotube's desirable intrinsic properties. Techniques, such as physical mixing, that have been successful with larger scale additives to polymers, such as glass fibers, carbon fibers, metal particles, etc. have failed to achieve good dispersion of CNTs. Two common approaches have been used previously to disperse the SWNTs in a host polymer:
1) Dispersing the SWNTs in a polymer solution by lengthy sonication (up to 48 h, M. J. Biercuk, et al., Appl. Phys. Lett. 80, 2767 (2002)), and
2) In situ polymerization in the presence of SWNTs.
Lengthy sonication of approach 1), however, can damage or cut the SWNTs, which is undesirable for many applications. The efficiency of approach 2), is determined by the degree of dispersion of the nanotubes in solution which is very poor and is highly dependent on the specific polymer. For example, it works better for polyimide (Park, C. et al., Chem. Phys. Lett., 364, 303(2002)) than polystyrene (Barraza, H. J. et al., Nano Ltrs, 2, 797 (2002)).
Although CNTs have exceptional physical properties, incorporating them into other materials has been inhibited by the surface chemistry of carbon. Problems such as phase separation, aggregation, poor dispersion within a matrix, and poor adhesion to the host must be overcome.
A process of noncovalent functionalization and solubilization of carbon nanotubes is described by Chen, J. et al. (J. Am. Chem. Soc., 124, 9034 (2002)) which process results in excellent nanotube dispersion. SWNTs were solubilized in chloroform with poly(phenyleneethynylene)s (PPE) along with vigorous shaking and/or short bath-sonication as described by Chen et al. (ibid) and in U.S. patent application US 2004/0034177 published Feb. 19, 2004, having U.S. Ser. No. 10/255,122, filed Sep. 24, 2002, and U.S. patent application U.S. Ser. No. 10/318,730 filed Dec. 13, 2002; the contents of such patent applications are incorporated by reference herein in their entirety. Composites of such functionalized and solubilized carbon nanotubes with the host polymers polycarbonate or polystyrene were fabricated and certain mechanical properties of such composites were reported in U.S. patent application US 2004/0034177 published Feb. 19, 2004, U.S. Ser. No. 10/255,122, filed Sep. 24, 2002, and in U.S. patent application U.S. Ser. No. 10/318,730 filed Dec. 13, 2002; the contents of which are incorporated by reference herein in their entirely.
The present inventors have addressed the problem of nanocomposites having nonuniform dispersion of nanomaterials in host polymer matrices that cause undesirable consequences to the composite material such as loss of strength, particle generation, increased viscosity, loss of processability, or other material degradation, and provide herein nanocomposites having improved properties.